The present invention relates generally to crystal growing apparatus used in growing monocrystalline ingots, and more particularly to a cable assembly for use in such crystal growing apparatus.
Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski ("Cz") method. The growth of a crystal ingot is most commonly carried out in a crystal pulling furnace or crystal puller. In this method, polycrystalline silicon ("polysilicon") is charged to a crucible and melted by a heater surrounding the outer surface of the crucible side wall. A cable assembly comprised of a shank and a cable wound on a drum supports a seed chuck capable of holding a seed crystal therein. The seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by slow extraction via the cable assembly. After formation of a neck is complete, the diameter of the crystal ingot is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the pull rate and heat supplied to the crucible. When the diameter becomes small enough, the ingot is then separated from the melt.
Conventional cable assemblies used in crystal pullers comprise a tungsten cable and a chuck support constructed of a malleable material, such as a non-magnetic austenitic stainless steel or a malleable tungsten alloy. The chuck support includes a shank swaged to the end of the cable extending within the crystal puller and a ball, sized larger than the shank, attached to the end of the shank opposite the cable. The chuck support is malleable so that the shank can be swaged on the end of the cable to connect the chuck support to the cable. The seed chuck is supported by the ball of the chuck support and hangs down therefrom such that the ball of the chuck support carries the weight of the seed chuck and growing ingot. The weight is then distributed up the shank to the cable.
Recent advances in crystal puller design and semiconductor wafer processing have led to more severe mechanical and thermal loads on the cable assembly. The creep rupture strength of the material used to form the chuck support is not sufficiently great to withstand the environment in the new puller. The chuck support tends to fail at the crosssection between the ball and the shank at mechanical and thermal loads well below loads that would cause the tungsten cable to fail. The chuck support has thus become the limiting factor in the usage lifetime of the cable assembly for growing silicon ingots. More particularly, the lifetime of the conventional cable assembly has been cut in half, from being useful in growing about twenty ingots down to being used in the growth of only about ten silicon ingots. The thickness of the shank cannot be increased to overcome this limitation because the shank must remain sufficiently malleable for swaging to the cable to form a strong joint. For similar reasons, the conventional chuck support cannot be constructed of near pure tungsten, which has a higher creep rupture strength under the severe thermal loading within the crystal puller, because it is not sufficiently malleable for swaging the shank to the cable.